This invention relates to a method and apparatus for coating a substrate and, in particular, to a system for automatically controlling the average, minimum, and distribution of coating thickness of material produced in a continuous-coating process. The application discussed is a continuous-galvanizing line. However, the system can generally be applied to any continuous-coating process with actuators that control the coating thickness and a gauge that measures the coating thickness. The system provides a means of maintaining a given average coating weight over the width of the sheet while ensuring that a given minimum-coating weight over a spot less than the width of the sheet will also be met. The system also provides a means for periodic logging of process data such as these average- and minimum-coating weights.
In a typical coating operation of the type to which this invention is directed, the strip material, such as steel, tin, aluminum, etc., is moved from an entry station, which includes a weld machine for welding front ends of a new coiled strip to the rear end of a previous strip, through cleaning tanks, and then to a storage facility consisting of an entry loop section. The simplest type of entry loop consists basically of a single loop of the strip which falls into a loop pit which may typically be 50 feet deep. Alternatively, the storage loop may include sets of stationary rollers and corresponding sets of movable rollers mounted on a "loop car." The loop car is motor driven to allow the loop to be shortened or lengthened as appropriate. From the loop storage facility the strip may proceed through an annealing furnace to the coating bath, such as a molten zinc bath having a nominal temperature of about 900.degree. F.
From the coating bath the coated strip travels upwardly in free travel a distance sufficient to cool the coating material. During this travel, the coated strip is moved past a pair of air knives located on each side of the moving strip. These air knives force air under pressure against the coated surface to adjust the coating thickness by forcing excess coating material downward toward the bath. The air stream is normally directed at a downward angle relative to the strip and the adjustment of this angle is generally made according to the operator's preference. The air knives are also capable of movement toward and away from the strip and the pressure of the air stream is also adjustable. It has been found that the system operates most effectively by maintaining equal pressures on both sides of the strip; therefore any adjustments to the knife pressure are made equally to both knives.
From the coating station, the strip travels to a measuring station which may be several hundred feet from the air knife location; this is the closest point to the coating bath that provides an environment in which the measuring apparatus can operate effectively.
Conventionally, coating weight measurements are made by taking test coupons from the strip and measuring the coating weight on each coupon. In compliance with ASTM (American Society for Testing and Materials) requirements, three coupons, or test slugs, are punched from the strip; the coupons are generally about 21/2 inches in diameter and are taken from the center of the strip and from locations approximately 2 inches in from each edge. The slugs are weighed, then the zinc coating removed by an acid bath treatment, and the stripped slugs again weighed in order to determine the coating weight on the strip.
Two ASTM requirements must generally be met: (1) the average coating weight for all of the coupons must be not less than a fist minimum average total coating weight; and (2) the coating weight for each coupon must be not less than a second minimum average coating weight. For the ASTM G-90 specification, for example, the minimum average total coating weight is 0.90 oz/ft.sup.2 and each coupon must have a coating weight of at least 0.80 oz/ft.sup.2. In addition, in some cases, at least a prescribed percentage of the total coating must be on each side of the test coupons. However, because of inherent system limitations, it is almost impossible to obtain an exactly uniform coating across the width of the strip from one edge to the other. In general, the edge coating thickness will be less than the specified minimum average total weight and coating thickness increases toward the center of the strip.
The principal object of the present invention is to provide an automatic control over the coating operation so as to minimize the amount of coated strip material rejected either by line inspectors or by the customer due to insufficient coating weight or improper coating distribution. Moreover, this automatic control operation must be performed on a strip moving at a rate of anywhere from 200 to 1200 or more feet per minute; in the particular line for which this invention was initially developed, the strip moves at a rate of about 600 feet per minute.